Professor Kaminsky, et al reported in 1976 that olefins may be polymerized using a methylaluminoxane (MAO) compound as a cocatalyst and a zirconocenedichloride compound as a catalyst (A. Anderson, J. G. Corde, J. Herwig, W. Kaminsky, A. Merck, R. Mottweiler, J. Pein, H. Sinn, and H. J. Vollmer, Angew. Chem. Int. Ed. Engl., 15, 630, 1976).
U.S. Pat. No. 5,324,800, filed by Exxon, describes olefin polymerization using a metallocene compound comprising a cyclopentadienyl ligand having a variety of substituents.
Metallocene catalysts have a uniform distribution of active sites, and thus, when using them in the production of a polymer, the distribution of molecular weight of the polymer obtained is narrow, the copolymerization of the polymer can be easily performed, and the distribution of a second monomer is uniform. Further, when using metallocene catalysts in the polymerization of propylene, a stereostructure of the polymer can be controlled according to the symmetricity of a catalyst. When Ziegler-Natta catalysts are used, only isotactic polypropylene can be prepared, but when metallocene catalysts are used, various polypropylenes, for example, isotatic, syndiotactic, atactic, and hemiisotactic polypropylenes, can be stereoregularly prepared. For example, syndiotactic polypropylene synthesized using a metallocene has low crystallinity, appropriate rigidity and hardness, and high transparency and impact resistance. That is, when the metallocene catalysts are used in the preparation of polyolefins, conformation of the polyolefins can be controlled and physical properties of the polymers can be easily controlled. Thus, vigorous research has been conducted on metallocene catalysts.
However, the technique of olefin polymerization using a homogeneous catalyst cannot be used in a gas phase process or a slurry process, since the shape of the polymer cannot be easily controlled. Further, an excess amount of MAO is required to maximize the activity of the metallocene catalyst. In order to overcome these problems, the metallocene catalyst should be supported on an appropriate carrier. The supported metallocene catalyst can control the shape of the obtained polymer and control the molecular weight distribution according to its applications. Further, the supported metallocene catalyst can increase an apparent density of the obtained polymer and reduce fouling in the reactor.
In general, conventional methods of preparing a supported metallocene catalyst include a method including chemically and physically binding a metallocene compound to a carrier and then contacting the resultant product to aluminoxane, a method including supporting aluminoxane on a carrier and then reacting the resultant product with a metallocene compound, a method of contacting a metallocene compound with aluminoxane and then supporting the resultant product on a carrier, etc. In order to have the same high activity and copolymerization efficiency as the homogeneous catalyst, the supported catalyst should maintain a single active site structure after being supported. In order to prevent the reactor fouling, the catalyst must not be separated from the carrier during the polymerization. The particle size, particle size distribution, and apparent density of the polymer depend on the particle shape and the mechanical properties of the supported catalyst.
Korean Laid-Open Patent Publication No. 10-0404780 describes a metallocene compound having a silacycloalkyl substituent and a supported catalyst using the compound. However, when the supported catalyst is used in a gas phase process or a slurry process, the catalyst is separated from the carrier, and thus, may induce reactor fouling.
Japanese Laid-Open Patent Publication No. Hei 6-56928 describes a method of preparing a supported metallocene catalyst, including supporting a ligand on a surface of a carrier via a chemical bond and then, binding metal to the ligand. This method is very complicated and a large amount of the catalyst cannot be easily supported on the carrier.
The method including supporting aluminoxane on a carrier and then reacting the resultant product with a metallocene compound among the above methods is the oldest method of preparing a heterogeneous catalyst having a single active site. For example, silica can be reacted with a solution of aluminoxane and filtered to obtain a filtrate and the filtrate can be reacted with zirconocene dissolved in toluene or an aliphatic hydrocarbon solvent, thereby preparing a supported catalyst. The obtained supported catalyst can be directly used in the polymerization or copolymerization of ethylene performed in a gas phase process or a slurry process. In this method, the cocatalyst is physically/chemically secured on a surface of the carrier and the catalyst forms an ion bond with the cocatalyst like a homogeneous catalyst, and thus, the catalytic activity is relatively high. Further, since a single phase catalyst can be prepared without further adding aluminoxane in the polymerization reactor, this method can be easily applied in a conventional gas phase or slurry process. However, separation of the catalyst from the carrier cannot be completely prevented, and thus reactor fouling can occur. Also, the amount of aluminoxane that can be bound to silica is limited, and thus, the amount of the metallocene compound that can be bound to aluminoxane is limited.